This invention relates generally to manufacture and activation of metal-containing catalysts and their use in hydrocarbon conversion reactions. More particularly, this invention concerns a method for making and activating a metal or metal carbide--metal oxide catalyst especially useful for promoting hydrogenation reactions in general and the methanation reaction in particular.
The transition metals of Group VIII of the Periodic Table have long been known for their catalytic activity when prepared in a finely divided or high surface area form. Nickel in particular has found extensive use as a catalyst in hydrogeneration and methanation reactions. A number of standard techniques to obtain high surface area metal catalysts have been developed. Illustrative of these are impegnation, precipitation, ion-exchange, Raney metal and reduction of the fused oxide. Since reactions which are promoted with a catalyst usually occur on the surface of the catalyst through adsorption of one or more of the reactants, it is usually desirable to obtain catalysts with the largest active surface area practicable.
When impregnation techniques are used to prepare a catalyst, a compound which is usually a salt of the desired catalytic elements is dissolved in a liquid. A high surface area support material such as kieselguhr, alumina, or activated charcoal is wetted with the solution and the mixture is thereafter dried and calcined. If a catalytic element in its metallic state is desired, the calcined mixture is thereafter reduced using a gaseous reductant such as hydrogen. Boyd et al in U.S. Pat. No. 2,666,756 disclose impregnation techniques for the preparation of metallic nickel or cobalt catalysts promoted with other compounds including the oxides of cerium and thorium supported on Kieselguhr and silica gel. Their catalyst finds use for the polymerization of ethylene.
Catalysts may be prepared by precipitation techniques in which a catalytic metal is precipitated from solution, generally as the hydroxide, either alone or with a carrier compound. The resulting precipitate is washed and dried and the metal compound is thereafer reduced to elemental form. Very small catalyst particles having a large surface are produced by this technique.
Taylor et al, in U.S. Pat. Nos. 3,395,104 and 3,404,100 disclose catalysts prepared by coprecipitating nickel and alumina. The precipitated mixture was thereafter impregnated with a solution containing either a rare earth metal or yttrium and the resulting mixture was dried and calcined. After reduction with hydrogen, the catalyst was used to react steam with naphtha vapors to produce a methane-rich gas.
Catalysts may be prepared by ion-exchange with a high surface area support having a large number of protonic sites. A solution containing cationic metal is added to a stirred slurry of the support material in a liquid to effect ion-exchange at the protonic sites. Thereafter, the support is washed and dried and the exchanged metal may be reduced. This results in metallic sites dispersed over the large surface of the support.
A skeletal high surface transition metal catalyst may be prepared through the use of Raney alloys. An alloy is obtained by melting a transition metal such as iron, cobalt or nickel with a soluble metal such as aluminum or zinc. The cooled alloy is comminuted to a convenient particle size and then leached in a heated alkaline solution to dissolve the aluminum or zinc. The resulting catalyst has a large skeletal surface area of active metal and finds use in hydrogenation and methanation reactions.
Another technique for preparing catalysts is by reduction of a fused oxide. An oxide of an easily reducible metal is fused with a promoter or support material which may comprise an oxide of a difficulty reducible metal. The cooled material is ground, sized, and reduced with hydrogen at temperatures sufficiently low to prevent sintering. There is obtained a porous bulk material having a high surface area interlaced with oxide promoters.
While all of these techniques produce useful and active catalysts, no one of the prior art methods combines the advantages of high surface area, ease of preparation and activation, high specific reactivity and overall economy as do the catalysts of this invention.